Luminescent polymers showing photoluminescence as well as polymers showing electroluminescence were proposed to be used in light emitting devices and electrooptical display elements.
The organic light emitting devices or diodes (OLEDs) currently being under intense research consist of at least one emission layer. Common OLEDs are realized using multilayer structures, where an emission layer is sandwiched between one or more electron-transport and/or hole-transport layers. By applying an electric voltage electrons and holes as charge carriers move towards the emission layer where their recombination leads to the excitation and hence luminescence of the lumophor units contained in the emission layer. The sandwich structure is built by vacuum deposition or spin coating techniques which may include a polymerization step before applying the next layer (Meerholz et al., Synthetic Metals 111-112 (2000) 31-34). OLEDs which are available in different colors have the potential of being used as the building blocks of different kind of information displays.
Also anisotropic luminescent polymers are known where the polymer and/or lumophor units are oriented. These emissive materials show anisotropic absorption and/or anisotropic emission of polarized light. The degree of absorption and/or emission of linearly polarized light depends on the relative orientation of the wavevector to the main director of the fluorophor molecules. Such an orientation within the luminescent materials can be achieved by different methods:                incorporation of luminescent molecules into an oriented polymer prior or after the orientation step,        tensile orientation of a ductile luminescent polymer (e.g., the techniques described in WO 00/07525),        rubbing of the luminescent polymer,        applying the Langmuir-Blodgett technique,        oriented growth of the luminescent materials onto oriented substrates, like onto known alignment layers,        polymerization of oriented liquid crystals,        photo-induced alignment,        alignment in electric, magnetic or flow fields.        
By using their anisotropic optical characteristics, these materials can replace polarizers and/or color filters which reduce the light efficiency in liquid crystal displays (LCDs) by up to 80% and more. Hence, display devices employing such anisotropic luminescent polymers are described to show a high brigthness and contrast, and furthermore a good viewing angle (Weder et al., Science 279 (1998), 835 and EP 889 350 A1). Using pixel elements of at least three different photoluminescent materials multicolor images may be displayed. In major embodiments of such display devices an anisotropic photoluminescent layer substitutes the polarizer of a conventional backlight—polarizer—light valve—polarizer arrangement, where the light valve uses known electrooptical effects of liquid crystal materials, like the TN- or ECB-effect. A high degree of polarized emission is necessary in embodiments where the photoluminescent layer is arranged directly behind the backlight. Whereas a high degree of polarized absorption is mandatory in devices where the photoluminescent layer is placed behind the light valve.
Methods and compounds to achieve charge transport properties and polarized luminescence from oriented materials as well as their application in displays are reviewed by M. Grell and D. D. C. Bradley, Adv. Mater. 1999,11, 895-905.
A proposed type of display uses polarized electroluminescence as background illumination of LCDs. Luessem et al. (Liquid Crystal 21 (1996), 903) report the fabrication of polymer based LEDs showing polarized electroluminescence. The orientation of the molecules within the light emitting layer was accomplished by the self organization of liquid crystal polymers (LCPs) deposited onto a rubbed polyimide film serving as an alignment layer.
In addition, organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation, i.e., it has a high ionisation potential, as oxidation leads to reduced device performance.
Compounds, especially dyes, comprising a 2-vinylene-oxadiazole group are known. The documents EP 0 360 618 A2, EP 0 458 325 A1 and DE 38 12 278 C2 are related to photosensitive compositions and describe oxadiazole derivatives as compounds capable of generating acids through irradiation of actinic or radiant rays. The given examples relate to 2-styryl-, 2-(alkylstyryl)-, 2-(alkoxystyryl)-, 2-(4-chlorostyryl)-, 2-(4-styrylstyryl)-, 2-(benzofuran-2-yl)-derivatives of 5-trichloro- and 5-tribromomethyl-oxadiazoles.
The JP 2000/290284-A proposes silane compounds for light-emitting devices. A diphenylsilane containing two 5-phenyl-2-styryl-oxadiazole groups is disclosed among other examples.
The U.S. Pat. No. 5,457,004 provides a high dye-yield coupler for photographic silver halide emulsion layers. The dye group within the coupler may contain a styryl-oxadiazole group.
An aim of this invention is to make available oxadiazole derivatives, which show a mesophase, preferably in a wide temperature range, which are chemically and photochemically stable, and/or stable against an electrochemically induced degradation, and/or which exhibit luminescent and/or charge transport properties.
Another aim of this invention is to make available luminescent oxadiazole derivatives, which themselves or in a liquid-crystalline mixture or in a polymerizable material are especially suitable for the production of anisotropic luminescent materials, especially polymers, showing advantageous anisotropic optical characteristics.
Another aim of the invention is to provide a liquid crystalline mixture, which shows a mesophase in a wide temperature range, which is chemically and photochemically stable, and/or stable against an electrochemically induced degradation, and/or which exhibits luminescent and/or charge transport properties.
Furthermore, it is an aim of the present invention, to provide a polymerizable material, which especially is suited for the production of polymer materials, which exhibit luminescent and/or charge transport properties.
Thus, it is another aim of the present invention, to provide a polymer material, which exhibits luminescent and/or charge transport properties.
Furthermore, it is an aim of the present invention to make available luminescent as well as anisotropic luminescent polymer materials with the above mentioned characteristics.
Further aims of the invention are to extend the pool of mesogenic or liquid crystalline compounds and mixtures, of polymerizable compounds, mixtures or materials, of polymer materials with charge transport and/or luminescent properties and of semiconductor, charge transport, photo-conducting, photo-luminescent and/or electro-luminescent materials available to the expert.
A further aim of this invention is also to show advantageous uses of these oxadiazole derivatives, liquid crystalline mixtures, polymerizable materials and polymer materials.
Further aims of the invention relate to a field effect transistor, a security marking or device and to a liquid-crystal display element.
Another aim of the present invention is to provide a charge injection layer, planarising layer, antistatic film or conducting substrate or pattern for electronic applications or flat panel displays.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
Definition of Terms
The term luminescence means emission of electromagnetic radiation, preferably in, but not limited to, the visible spectrum, due to any kind of excitation, preferably by electromagnetic radiation (photoluminescence) or by an applied electric voltage (electroluminescence). The more general term luminescence encompasses phosphorescence and fluorescence, the latter being the preferred meaning.
The term mesogenic group means a rod-shaped, lath-shaped or disk-shaped group, i.e., a group with the ability to induce liquid crystal phase behaviour.
The terms mesogen and mesogenic, liquid crystal and liquid crystalline compound as used in the foregoing and the following comprise compounds with a least one mesogenic group. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerised.
The term ‘film’ includes self-supporting, i.e., free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.